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An Interactive PZT Phase Diagram. To use, click on the different regions of the diagram.
The cubic structure, found at high temperatures, does not possess a spontaneous dipole.
The cubic structure means that there is no movement of the Ti ion inside the O octahedra. This results in no net dipole moment, making cubic PZT non-piezoelectric.
In the tetragonal phase, the most important atom motion is that of the Ti atom in the O octahedra, in the originally [100] direction.
The atom forms a dipole moment in the [100] direction. This gives it 6 possible directions to form, passing through each vertex of the octahedra.
In the rhombohedral phase, the Ti atoms move in the [111] direction. The unit cell has the same lattice parameter along each axis, so there is a movement which is symmetrical about the [111] direction.
The movement of the Ti atom forms a dipole moment in the [111] direction. This gives it 8 possible dipole moment directions, through the centre of each octahedral face.